This invention relates to video signal dropout compensators providing compensation for luminance and chrominance signal components.
The broadcasting of prerecorded television programs is often complicated by the presence of defects in the broadcast signal that were not present in the original signal when it was recorded. When the defect results in the loss of the video signal or the presentation of an incorrect signal, the defect is termed a signal dropout. When the recording medium is video tape, the dropout can be caused by an accumulation of dirt on the recorder heads or on the tape itself, or by a defect in the manufacture of the tape. With recordings on video discs, defects, such as manufacturing imperfections or scratches in the disc surface or dirt accumulation in the record grooves, can result in the occurrence of a signal dropout. The dropout may appear on the television receiver as a light or dark spot or streak. If the recording contains a large number of dropouts, the displayed picture can be very annoying to the viewer. To eliminate the annoying aspects of these signal dropouts, a form of dropout compensation is needed.
All dropout compensators generate a replacement signal, but they can operate in several different ways. One type of dropout compensator merely replaces every signal dropout with an average value replacement signal, which appears as gray, based on the theory that an average gray picture signal will come close to approximating the actual television picture that was lost in most dropout instances and will therefore present a much less annoying picture to the viewer. This type of dropout compensation, however, presents problems when the dropout occurs in a picture area that is either very light or very dark, making the gray replacement signal a poor approximation. Even poorer results occur when color television signals are involved, as the gray dropout replacement is even more noticeable in color than with the monochrome picture. Since almost all programming today is in color, this method is seldom used.
A popular method of dropout compensation replaces the defective signal with video information from the corresponding location on the previous television line. This method works fairly well in theory because the television information is largely redundant from one line to the next. Problems arise in practice however, due to the fact that standard color television signals employ a phase shift of the chrominance signal component between each television line in a television field. In the NTSC system, the chrominance phase shift between lines of a given field is 180.degree.. Merely replacing defective information with video information from the previous line would result in a replacement signal having its chrominance component 180.degree. out of phase, giving a complementary color display, which is objectionable to the viewer.
Replacement information may be taken within the same field from the second line prior to the dropout line to provide proper chrominance phase of the replacement signal, but the degree of dropout signal approximation is greatly reduced. Because of field interlace, information from two lines before the dropout line in a given field seems to come from four lines before when viewed on a TV receiver. In order to use the previous line information from a given field, the chrominance phase must be changed by 180.degree.. A common technique used in prior art dropout compensators was to filter the video signal to separate the luminance and chrominance components, invert the chrominance component to change its phase by 180.degree. and then recombine the luminance and phase-shifted chrominance to give a dropout replacement signal. This method is satisfactory as long as the theory that video information is redundant from line to line is correct. Often the nature of the particular television picture results in incorrect dropout replacement when only information at the same horizontal location from the previous line is used. U.S. Pat. No. 4,122,489 of Bolger et al. describes a method in which information from both the lines previous and subsequent to the dropout line in used to form a replacement signal. Although Bolger's method uses the video information which occurs at horizontal locations in the television lines before and after the dropout location, information at the corresponding horizontal dropout location in the preceding and subsequent lines is ignored. Also, Bolger's technique develops the information from information locations occurring at two samples before and two samples after the dropout point, so that the averaged replacement value has the proper chrominance phase. This somewhat remote selection of samples for a replacement signal tends to degrade the quality of the dropout signal approximation.
It would be advantageous to provide a video signal dropout compensator which generates a dropout replacement signal which is a closer approximation to the signal dropout than the described prior art.